Pari

In the episode description of the episode "Vecna's Curse", it plainly states that a dead body brought Hawkins to a halt. Meaning, a dead body caused time to stop. THIS is likely why time stopped in the Upside Down Hawkins on November 6th- Will died that day. This is also likely the event that created everything- including the Upside Down itself. Someone's world literally turned upside down because of Will's death.

Mike likely had a hand in stopping/reversing/altering reality to prevent this from occurring... especially since we are shown a visual clue of time coming to a halt that day from a wheel stopping.

Max refuses to spend "the last day of [her] life" in "Mike Wheeler's basement" which is exactly where Will spent the last day of his life. No throwaway lines.

Mike always feels like he keeps losing Will because, in some reality, he did.

Will's entire move to California is symbolic for death. Will moved west, where the sun sets, to Lenora which is a reference to the "Lost Lenore" trope.

The episodes "Dear Billy" and "The Nina Project" have SO many clues alluding to Will's death.

Max reads her letter to a deceased William with the song "Letter to Willy" playing. We are shown graves of both "William" and "_YERS".

At Pennhurst, Hatch says:

"We found that music has a particularly calming effect on the broken mind. The right song, particularly one which holds some personal meaning, can prove a salient stimulus. But there are those who are... beyond a cure."

While he says this, we are shown these particular songs on the blackboard. Each one of these songs are about dreaming of another reality and reconnecting with a lost love! More on that here.

In "The Nina Project", the song "Letter to Willy" plays again while our characters are digging a grave for a hero. Mike and Will take a break to have a heart-to-heart.

During that heart-to-heart, we are shown a clear reference to the very first episode, the very last moment between Will and Mike prior to Will's disappearance. Mike expresses frustration with himself, thinking he could have prevented El from being taken. It's actually about Will.

The whole Mike is "the heart" while we are given lots of references to hearts being broken, stabbed etc are likely to due Mike's grief. Mike has a hole in his heart.

The story of Nina is important. Nina's lover was killed and she buried the memory... just like Mike. Mike is Nina.

Mike was so traumatized that he buried the memory of Will's death... believing he is still alive this entire time. Waiting for his return. He is stuck in a loop. He lost his mind.

Good news though. While Papa intentionally omits the second part of the play, there is a happy ending for Nina.

Much like Nina, there is a cure to Mike's insanity... his beloved. Which turns out not to be El but...